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[thirdparty/man-pages.git] / man2 / vmsplice.2

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.TH VMSPLICE 2 2006-04-28 "Linux" "Linux Programmer's Manual"
.SH NAME
vmsplice \- splice user pages into a pipe
.SH SYNOPSIS
.nf
.B #define _GNU_SOURCE
.B #include <fcntl.h>
.B #include <sys/uio.h>

.BI "long vmsplice(int " fd ", const struct iovec *" iov ,
.BI "              unsigned long " nr_segs ", unsigned int " flags );
.fi
.SH DESCRIPTION
.\" Linus: vmsplice() system call to basically do a "write to
.\" the buffer", but using the reference counting and VM traversal
.\" to actually fill the buffer. This means that the user needs to
.\" be careful not to re-use the user-space buffer it spliced into
.\" the kernel-space one (contrast this to "write()", which copies
.\" the actual data, and you can thus re-use the buffer immediately
.\" after a successful write), but that is often easy to do.
The
.BR vmsplice ()
system call maps
.I nr_segs
ranges of user memory described by
.I iov
into a pipe.
The file descriptor
.I fd
must refer to a pipe.

The pointer
.I iov
points to an array of
.I iovec
structures as defined in
.IR <sys/uio.h> :

.in +4n
.nf
struct iovec {
    void  *iov_base;            /* Starting address */
    size_t iov_len;             /* Number of bytes */
};
.in
.fi

The
.I flags
argument is a bit mask that is composed by ORing together
zero or more of the following values:
.TP 1.9i
.B SPLICE_F_MOVE
Unused for
.BR vmsplice ();
see
.BR splice (2).
.TP
.B SPLICE_F_NONBLOCK
.\" Not used for vmsplice
.\" May be in the future -- therefore EAGAIN
Do not block on I/O; see
.BR splice (2)
for further details.
.TP
.B SPLICE_F_MORE
Currently has no effect for
.BR vmsplice (),
but may be implemented in the future; see
.BR splice (2).
.TP
.B SPLICE_F_GIFT
The user pages are a gift to the kernel.
The application may not modify this memory ever,
.\" FIXME Explain the following line in a little more detail:
or page cache and on-disk data may differ.
Gifting pages to the kernel means that a subsequent
.BR splice (2)
.B SPLICE_F_MOVE
can successfully move the pages;
if this flag is not specified, then a subsequent
.BR splice (2)
.B SPLICE_F_MOVE
must copy the pages.
Data must also be properly page aligned, both in memory and length.
.\" .... if we expect to later SPLICE_F_MOVE to the cache.
.SH RETURN VALUE
Upon successful completion,
.BR vmsplice ()
returns the number of bytes transferred to the pipe.
On error,
.BR vmsplice ()
returns \-1 and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I fd
either not valid, or doesn't refer to a pipe.
.TP
.B EINVAL
.I nr_segs
is 0 or greater than
.BR IOV_MAX ;
or memory not aligned if
.B SPLICE_F_GIFT
set.
.TP
.B ENOMEM
Out of memory.
.SH VERSIONS
The
.BR vmsplice ()
system call first appeared in Linux 2.6.17.
.SH "CONFORMING TO"
This system call is Linux specific.
.SH NOTES
.BR vmsplice ()
follows the other vectorized read/write type functions when it comes to
limitations on number of segments being passed in.
This limit is
.B IOV_MAX
as defined in
.IR <limits.h> .
At the time of this writing, that limit is 1024.
.SH SEE ALSO
.BR splice (2),
.BR tee (2),
.BR feature_test_macros (7)